The present invention relates to a synthetic polymer composition useful as the binder component in coating colors and to coating colors containing these polymeric compositions.
In the production of paper, the paper surface is often coated with a composition (commonly referred to as a coating color) which imparts desirable properties such as printability to the paper. Conventionally, the coating color consists primarily of suspension of a pigment and/or filler such as clay in an aqueous medium containing a binder.
In one conventional method for applying the coating color, the coating is continuously transferred as a liquid film from an applicator roll to the paper surface, with any applied excess removed using suitable means such as blade or air-knife techniques. In the preparation of the highest quality paper, the paper is often coated sequential with a coating color of one composition and thereafter with a coating color of a second composition. In general, in the preparation of the double coated paper, the two coating colors contain the same or similar binders but different filler materials.
For various reasons, paper producers have continuously strived to reduce the finished weight of the coated paper. One method by which this can be achieved is by reducing the amount of the coating color applied to the paper. Unfortunately, a reduction in the coat weight generally results in a drop of the finished paper properties, particularly in the print quality. To effectively coat the paper, at the desirable coat weight, the coating colors advantageously exhibit desirable physical properties, e.g., stability and rheological properties. These properties are particularly important in the preparation of quality paper grades such as those printed by gravure techniques.
Heretofore, a natural, high molecular weight material such as starch or protein has been used as the binder component of the coating color. Unfortunately, these natural materials are susceptible to attack by microorganisms and when employed alone give brittle coatings. Moreover, using a starch binder, the coated paper often does not possess the required print quality due to insufficient coat hold-out, i.e., excessive penetration of the coating into the paper.
It has heretofore been suggested to employ synthetic polymers as the binder in paper coatings. Many such synthetic polymer binders consist of two polymeric components with one copolymer being employed primarily to impart the desired binding strength and other properties to the coated paper and the second copolymer component being employed primarily to affect the rheological properties of the coating colors prepared therefrom. For example, German Patent No. 1,546,315 discloses a synthetic polymer binder comprising 60 to 95 percent of a first copolymer of butadiene, styrene and/or acrylonitrile and 5 to 40 percent of a second copolymer of acrylic or methacrylic acid, a monomer which forms a water-insoluble homopolymer (at least a portion of which is an ester of acrylic or methacrylic acid) and, optionally, acryl- or methacrylamide. Unfortunately, coating colors prepared from this binder are generally deficient, particularly using gravure printing techniques, due again to insufficient coat hold-out. In addition, using coating colors prepared from the described polymeric binders difficulties are experienced in the preparation of a double coated paper. Specifically, an undesirable number of paper breaks and discontinuities on the coated paper are experienced using the coating colors prepared from these copolymer binders.
A similar synthetic polymeric binder, except that the first copolymer is derived from an ester of acrylic or methacrylic acid, a vinyl ester or propionic acid, and optionally other copolymerizable monomers, is described by U.S. Pat. No. 3,365,410. While coating colors prepared using these polymeric binders improve the print quality of paper prepared therefrom, a further improvement in the balance of the paper properties is required.
In view of the aforementioned deficiencies of the synthetic polymer binders employed heretofore, it remains highly desirable to provide a polymeric composition which can effectively be employed as the binder in a paper coating color to impart an improved balance of properties in the paper coated therewith.